The reactivity of a neutron chain reaction in a nuclear reactor is a measure of the relative multiplication rate of neutron production. In a stationary state normal operation, the reactivity is zero. To increase the output, the reactivity may be increased and to reduce the output, e.g. in terms of the power generated, the reactivity may be reduced.
Generally speaking in the nuclear reactor and in the region in which a fissionable fuel, also referred to as a nuclear fuel, provides the neutron flux, i.e. the reaction region of the nuclear reactor, the reactivity is primarily controlled by the effectiveness of neutron absorbers. For example, neutron-absorbing rods or elements can be introduced into the core of a nuclear reactor, i.e. into the neutron flux zone, to a greater or lesser extent, or the fuel rods may be shifted into a body of neutron-absorbing material to a greater or lesser extent. The introduction and retraction of the absorber rods, for example, reduce or increases the reactivity.
Apart from the settings of the rods to control reactivity, the reactivity is influenced in normal operation by many other parameters. Of the greatest importance is the danger of uncontrolled liberation of neutrons in cases of uncontrolled reactivity with the consequence of potential danger of an explosion like increase of neutron production and loss of control of the reactor. This can be great enough to damage the reactor core and thus the most important barrier or containment preventing escape of radio-active substances.
For this reason it is desirable to provide an automatic inherently reliable reduction in the reactivity of a neutron-chain reaction which can be built into the reactor and can in a fail-safe manner effect at least a limited reduction in the reactivity upon the development of an event necessitating that reduction, such as a loss of coolant flow. A system which can reduce reactivity intrinsically and automatically as the need arises, contributes to the stabilization or stable operation of the nuclear reactor and enables hot shutdown thereof.
Some systems have been provided heretofore to generate a self-stabilizing effect in the operation of nuclear reactors. These can include the provision of a nuclear fuel having a negative reactivity temperature coefficient, or can utilize the negative reactivity void coefficient of the boiling water in a boiling-water reactor in which boiling water surrounds the fuel.
In such cases a tendency toward a temperature increase will automatically reduce the reactivity and thereby lower the output in a self-stabilizing manner.
These earlier self-stabilizing systems are, however, incompletely effective and it is desirable to be able to provide additional self-stabilizing automatic reactivity-controlling effects in a nuclear reactor.